Draft: Add randomized delay when syncing group project scan result policies

What does this MR do and why?

Adds a randomized delay of up to 10 seconds when enqueuing all projects affected by some group-level scan result policy. The reason is that a security configuration applies to a potentially large number of projects, and to process a given (project, configuration) pair, we need read the policy blob from Gitaly. To avoid hammering Gitaly with a large number of parallel reads, the randomized delay is added.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.